What is the difference between a hot runner and a cold runner mould?

A cold runner mould machines channels into the plates that carry melt to the cavities; the runner freezes and is ejected with the parts as scrap or regrind. A hot runner keeps the melt liquid inside heated manifolds and nozzles, delivering it directly into the cavity with no solid runner.

How much more does a hot runner mould cost?

A hot runner system typically adds from a few thousand euros for a simple single-drop system to €10,000-30,000+ for multi-drop valve-gate systems on larger tools, plus a temperature controller. Cold runner tools are simpler and cheaper to build and maintain.

When is a hot runner worth the investment?

When annual volume is high, resin is expensive, part weight is small relative to the runner, or gate cosmetics matter. The scrap and cycle-time savings usually repay the premium within high-volume production; for prototype and low volumes a cold runner is almost always the right choice.

Can cold runner scrap be reused?

Usually yes, as regrind — runners are ground and blended back with virgin resin, typically at 10-30% depending on the material and part requirements. Optical, medical and food-contact parts often forbid regrind, in which case a cold runner's runner is pure waste cost.

What are the downsides of hot runners?

Higher tool cost and complexity, longer colour-change and material-change purges, sensitivity to heat-degradable resins like PVC and POM if poorly designed, hot-drop vestige marks, and maintenance of heaters, thermocouples and seals. They also need skilled setup.

Which runner system does Nordmould use for low-volume production?

For Rapid and Bridge tier volumes, cold runner tools are normally quoted because their lower build cost wins at low volumes. At Production volumes Nordmould's quoting compares both systems on your part's scrap and cycle economics and prices the cheaper whole-life option.

Hot Runner vs Cold Runner Moulds: Cost, Scrap & When to Choose Each

The runner system is how molten plastic travels from the machine nozzle into the cavities. A cold runner freezes in the tool and is ejected with the parts every cycle; a hot runner stays molten inside the tool and never becomes scrap. The choice moves thousands of euros between tooling budget and running cost — which side wins depends on volume, resin price and part design.

How do the two systems compare?

Factor	Cold runner	Hot runner
Tool cost	Lower — simplest option	Higher: typically +€3,000-30,000+ depending on drops and gating
Scrap per cycle	Runner solidifies every shot (regrind or waste)	None — all melt becomes parts
Cycle time	Runner often dictates cooling time	Faster — only the part cools
Gate vestige (witness mark)	Larger; edge/tunnel gates leave visible marks	Small hot-tip dot; valve gate cleanest
Colour / material changes	Fast — purge and go	Slow — manifold holds melt, long purges
Heat-sensitive resins (PVC, POM, some FR grades)	Forgiving	Risky if manifold has dead spots
Cavitation balance	Natural in balanced layouts	Needs engineering; nozzle-to-nozzle variation possible
Maintenance	Minimal	Heaters, thermocouples, seals, controller
Automation friendliness	Runner must be separated/robotically handled	Parts drop clean — ideal for lights-out running
Best volume zone	Prototype to ~50k/yr typical	High volume, expensive resin, multi-cavity

How does the economics calculation actually work?

A cold runner's hidden cost is the runner itself: every cycle moulds resin you paid for and then throw away or regrind. If the runner weighs as much as the parts — common with small parts in a two-plate tool — you are buying roughly double the resin and cooling double the mass.

A hot runner's hidden cost is the premium plus maintenance, amortised over production. The breakeven question is: does the annual saving (scrap resin + cycle seconds + runner handling) exceed the annualised system premium? For a €0.10/part scrap penalty at 500k parts/yr, that is €50k/yr against a one-time €10-20k premium — the hot runner pays back in months. At 5k parts/yr it never does.

This is why the tiers split cleanly: Rapid and Bridge tooling is cold runner; Production tooling evaluates hot runner on the numbers. Nordmould prices both options on Production quotes where the volume justifies the comparison.

What about gate quality and part cosmetics?

Cold runner gates (edge, fan, tunnel) leave a visible witness where the runner was separated — acceptable on non-cosmetic faces, a problem on A-surfaces. Hot-tip gates leave a small dot, typically 0.5-1.5 mm, usually placed on a hidden face. Valve gates leave the cleanest mark and give independent control of each cavity's fill — the standard choice for large cosmetic parts and sequential filling.

If your part needs a gate on a visible surface, that constraint alone can decide the runner question. Raise gate placement in the DFM review; the gate types guide covers the options and their witness marks.

When should you specifically avoid a hot runner?

Three cases. First, frequent material or colour changes: the manifold holds melt that must be purged through, turning a 20-minute cold-runner changeover into hours. Second, heat-degradable resins in a poorly designed manifold — PVC, acetal and some flame-retardant grades degrade in dead spots and contaminate production with black specks. Third, low or uncertain volumes: paying a €10-30k premium against an unproven demand forecast is speculation, not engineering.